Not Applicable.
The present invention relates to a projection exposure lens in a projection exposure apparatus such as a wafer scanner or a wafer stepper used to manufacture semiconductor elements or other microstructure devices by photolithography and, more particularly, to a catadioptric projection optical lens with an object side catadioptric system and a refractive system for use in such a projection exposure apparatus.
U.S. Pat. No. 4,779,966 to Friedman gives an early example of such a lens, however the catadioptric system being arranged on the image side. Its development starting from the principle of a Schupmann achromat is described. It is an issue of this patent to avoid a second lens material, consequently all lenses are of fused silica. Light source is not specified, band width is limited to 1 nm.
U.S. Pat. No. 5,052,763 to Singh (EP 0 475 020) is another example. Here it is relevant that odd aberrations are substantially corrected separately by each subsystem, wherefore it is preferred that the catadioptric system is a 1:1 system and no lens is arranged between the object and the first deflecting mirror. All examples provide only fused silica lenses. NA is extended to 0.7 and a 248 nm excimer laser or others are proposed. Line narrowing of the laser is proposed as sufficient to avoid chromatic correction by use of different lens materials.
U.S. Pat. No. 5,691,802 to Takahashi is another example, where a first optical element group having positive refracting power between the first deflecting mirror and the concave mirror is requested. This is to reduce the diameter of the mirror, and therefore this positive lens is located near the first deflecting mirror. All examples show a great number of CaF2 lenses.
EP 0 736 789 A to Takahashi is an example, where it is requested that between the first deflecting mirror and the concave mirror three lens groups are arranged, with plus minus plus refractive power, also with the aim of reducing the diameter of the concave mirror. Therefore, the first positive lens is located rather near to the first reflecting mirror. Also many CaF2 lenses are used for achromatization.
DE 197 26 058 A to Omura describes a system where the catadioptric system has a reduction ratio of 0.75xe2x89xa6xcex21 less than 0.95 and a certain relation for the geometry of this system is fulfilled as well. Also many CaF2 lenses are used for achromatization.
For purely refractive lenses of microlithography projection exposure system a lens design where the light beam is twice widened strongly is well known, see e.g. Glatzel, E., Zeiss-Information 26 (1981), No. 92, pages 8-13. A recent example of such a projection lens with +xe2x88x92+xe2x88x92+ lens groups is given in EP 0 770 895 to Matsuzawa and Suenaga.
The refractive partial objectives of the known catadioptric lenses of the generic type of the invention, however, show much simpler constructions.
A catadioptric projection exposure lens comprising lenses or mirrors which are aspheric are known from JP 10-10429 and EP 0 869 383.
According to JP 10-10429 the aspheric surface is placed in the vicinity of a reflecting mirror.
By placing the aspheric surface in vicinity of the reflecting mirror, a good correction of distortions is achieved. Furthermore the system according to JP 10-10429 comprises an intermediate image.
From EP 0 869 383 a catadioptric system comprising at least two aspheric surfaces is known. For correcting off-axis-aberration one of the aspheric surfaces satisfies the condition
h/xcfx86 less than 0.85 
and for correcting on-axis-aberration the other of the aspheric surfaces satisfies the condition
0.85 less than h/xcfx86 less than 1.2 
whereby h is the height at each lens surface of the light beam that is assumed to be emitted from the intersection of the optical axis and the object plane and passes through the lens surfaces with the maximum numerical aperture NA and xcfx86 is the radius of the diaphragm of the aperture stop. Subject matter of EP 0 869 383 therefore is to ensure a high image quality by using aspheric surfaces.
Only as a point amongst others EP 0 869 383 mentions that by using aspheric surfaces the number of lenses in a catadioptric system can be decreased. Furthermore EP 0 869 383 relates only to systems with an intermediate image. As special embodiments EP 0 869 383 shows systems with the first aspheric surface placed near the intermediate image while the second aspheric surface is placed near the concave mirror of the catadioptric system or near the aperture stop.
WO 99/52004 shows embodiments of catadioptric objectives with few lenses, some of them being aspheric. From WO 99/52004 a system with 16 lenses, at least four of them being aspheric lenses and a numerical aperture of 0.65 is known.
From E. Heynacher, Zeiss-Information 24, pp. 19-25 (1978/79), Heft 88, it is known that with complicated optical systems it is less appropriate to treat imaging errors separately by aspheres, but to influence the correction of the imaging errors as a whole.
It is an object of the present invention to obtain a catadioptric optical system of new construction principles allowing for large numerical aperture, large image field, sufficient laser bandwidth, solid and stable constructions, which takes into account the present limitations on availability of CaF2 in quantity and quality. Therefore it is the major object of the present invention to minimize the number of lenses in a projection exposure lens for DUV (193 nm) and VUV (157 nm) systems. Furthermore said systems should not be restricted to systems with an intermediate image.
In order to achieve the above object, according to the present invention, there is provided a projection exposure lens according to claim 1.
It is a further object of the invention by minimizing the number of lenses to reduce the absorption and the reflection losses of the whole projection exposure lens.
Said further object is achieved by reducing the number of lenses in the second double passed lens system of the projection exposure lens since in the double passed lens system undesirable effects of absorption in the lens material and of reflection losses at the surface count twice.
According to the invention the second lens system comprises at maximum five lenses, preferably two or three lenses.
In a preferred embodiment of the invention negative refraction power is arranged in the second lens system between the optical elements for splitting beam and the concave mirror. Said negative refraction power is split into advantageously two negative lenses.
In a further preferred embodiment for correcting the chromatic length aberration CHL the second lens system provides for a over correction while the first and third lens system provides for a under correction.
The long drift section in the second lens system according to the invention provides for several advantages:
Mounting of the lens components in the second lens system is less complicated than in objectives known from the prior art.
The lenses of the second lens system and the concave mirror could be mounted as a separate lens group, no metallic tube is necessary between the optical elements for splitting beam and the first lens of the second lens system.
Further advantageous embodiments are obtained when including features of one or more of the dependent claims 4 to 61.
An advantageous projection exposure apparatus of claim 62 is obtained by incorporating a projection exposure lens according to at least one of claims 1 to 61 into a known apparatus.
A method of producing microstructured devices by lithography according to the invention is characterized by the use of a projection exposure apparatus according to the preceding claim 62. Claim 63 gives an advantageous mode of this method.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention. Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.